EMMU is a custom MMU chip for the 130XE Atari 8-bit computer. It is used to access the extra 64kB of RAM present in the 130XE.
Map of Emmu, 88405 P%C3%A4rnu County, Estonia
The user cannot directly manipulate the EMMU, but selects the various ROMS and memory banks via the memory-mapped hardware register known as PORTB (5401710 or D30116). Atari changed PORTB from an input port on the 400/800 machines to an output port on the XL/XE machines, leaving two joystick ports instead of four on the XL/XE machines.
By setting and clearing specific bits in PORTB, the user can access the extra 64kB of memory in 16kB increments. No synchronization is required as the OS handles the access.
The bit assignments for PORTB are as follows:
References
EMMU Wikipedia(Text) CC BY-SA